The invention relates to a control clamp for adjusting the flow rate of a liquid through a flexible hose, such as the hose of an infusion device, comprising a means for varying the flow cross section.
Such control clamps are known, for instance, from DE-OS No. 20 43 551, DE-PS No. 22 42 539, DE-OS No. 27 41 594, and U.S. Pat. No. 3,215,395.
Packages for one-time use have become widely accepted in the parenteral administration of liquids by infusion or transfusion devices. Such packages usually contain a hose of flexible material, such as polyvinylchloride, PVC, and a control clamp by means of which the flow cross section of the hose can be varied to adjust the velocity of flow of the liquid as desired. Typical flow velocities are from 50 to 120 ml/h which corresponds to about 25 to 60 drops/min. With known control clamps such flow velocities are adjusted by squeezing the hose by way of the control clamp until it has a gap width of approximately 4/100 mm.
Once the flow velocity has been adjusted for a liquid to be administered parenterally, it should not change involuntarily. Yet the hoses in question, particularly those made of PVC, as is well-known, have the property of becoming deformed plastically under pressure. This so-called "cold flow" of the plastic material in the long run results in a change of the flow cross section so that the required constant nature of the flow velocity cannot be assured.
DE-OS No. 20 43 541 already has for its object the provision of a control clamp with which cold flow is avoided. In his younger DE-OS No. 26 53 515, the same applicant confirms on page 4, line 6, that this aim has not been fully reached in that case.
According to German Pat. No. 22 42 539 the cold flow of the hose material is to be prevented by the asymmetric provision of a recess at one side of a clamping face. Into this recess a so-called excess of the hose is formed during squeezing, and it remains largely free of mechanical forces which might change its shape, even in clamped condition. For this reason it is said not to have any cold flow.
The flow velocity of the parenterally administered liquid is influenced not only by the flow cross section of the hose but by other parameters as well, such as the pressure conditions prevailing at both ends of the hose. Critical in this context are particularly pressure variations at the patient end as they may cause undesirable changes in the flow conditions. The known control clamps essentially adjust the flow cross section of the hose in a strictly limited range only. As a consequence of this "point action" of the known control clamps any pressure variation without dampening at the patient end directly changes the flow conditions, whereby the flow velocity varies undesirably.
Limiting the clamping action to a rather short hose section has another disadvantageous consequence in that the flow cross section may be altered by tensile forces acting in longitudinal direction of the hose.
U.S. Pat. No. 3,215,395 describes a control clamp which may serve to squeeze a certain length of hose. Yet the length of the distance in that case is limited by the circumference of a clamping roll of polygonal shape. Moreover, the clamping roll described requires some skill on the part of the user and, obviously, does not prevent cold flow.
It is the object of the invention to design a control clamp of the generic kind in question such that, whil preventing cold flow, the flow velocity of the liquid through the flexible hose remains constant, once adjusted, and is also largely uninfluenced by pressure variations at the patient end.